Increased rainfall over and downwind of urban areas is well documented and a surprisingly large effect. NASA on the subject.

NASA researchers have for the first time used a rainfall-measuring satellite to confirm that “urban heat-islands” create more summer rain over and downwind of major cities, including Atlanta, Dallas, San Antonio and Nashville.

NASA’s Goddard Space Flight Center, Greenbelt, Md., found that urban areas with high concentrations of buildings, roads and other artificial surfaces retain heat and lead to warmer surrounding temperatures, and create urban heat-islands. This increased heat may promote rising air and alter the weather around cities.

“Cities tend to be one to 10 degrees Fahrenheit [.56 to 5.6 Celsius] warmer than surrounding suburbs and rural areas and the added heat can destabilize and change the way air circulates around cities,” said Shepherd. Rising warm air may help produce clouds that result in more rainfall around urban areas.

NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite found that mean monthly rainfall rates within 30-60 kilometers downwind of the cities were, on average, about 28 percent greater than the upwind region. In some cities, the downwind area exhibited increases as high as 51 percent. It was also found that, on average, maximum rainfall rates in downwind regions often exceeded the maximum values in upwind regions by 48 percent – 116 percent. These results are very consistent with earlier related experiments in St. Louis, Missouri and near Atlanta.

While I do not disagree that UHI may have an effect, for Adelaide one should note that the heavier rain fell in the Adelaide Hills which always have had more rain than the flat Elizabeth area to the north west of Adelaide. Temperatures are lower at higher altitude and this can cause condensation of moisture in the atmosphere. Where rain falls is then a function of prevailing wind and weather direction in relation to higher altitude terrain.

Tom Harley:
The Adelaide Hills are wetter and cooler than the Plains. On very hot days there is little to separate the maximums but the Hills gets a lower minimum e.g. Wed 12. Adelaide 41.7 and dropped to 24.7 o/night.
Mt. Barker 39.1 dropping to 19.

Tuesday 11 was similar, Adelaide 19.5 to 42.5 and dropping to 29.0; Mt. Barker 11.5 to 40 then 19.4

The annual rainfall in the Hills is 50 to 120% higher than Adelaide itself.

Perhaps the “updraught” produced by the urban heat island simply draws in the clouds from further away. Once each “hot spot” has been doused, the clouds can drift in the direction of the next attractor.